1. Field of the Invention
The present invention relates to a photomask, a photomask blank, which is a raw material of the photomask, and fabrication methods thereof.
2. Description of the Related Art
In recent years, in order to meet the demand for miniaturization of circuit patterns required for increasing the packaging density of large-scale integrated circuits, advanced semiconductor micromachining techniques have become extremely important. For example, increasing the packaging density of a large-scale integrated circuit essentially requires a technique of thinning wires of wiring patterns in the circuit or a technique of miniaturizing a contact hole pattern for interlevel wiring of a cell. The trend toward miniaturization of circuit patterns of large-scale integrated circuits is being accelerated because it is the most effective approach to increase the operation speed thereof and reduce the power consumption thereof.
Most of such advanced micromachining techniques are based on the photolithography technique using a photomask. Therefore, the photomask, as well as the exposure apparatus and the resist material, is an essential technique for miniaturization. Therefore, in order to provide a photomask having a wiring pattern of thinned wires and a miniaturized contact hole pattern described above, development of a technique for forming a finer and more precise pattern on a photomask blank has been pursued.
To form a highly precise photomask pattern on a photomask substrate, it is essential that a resist pattern formed on a photomask blank is highly precise. When micromachining a semiconductor substrate, reduction projection photolithography is performed, and therefore, the size of the pattern formed on the photomask is about four times as large as the size of the pattern formed on the semiconductor substrate. However, this does not mean that the restriction on the precision of the pattern formed on the photomask is relaxed. On the contrary, the photomask pattern has to be formed with higher precision than the pattern provided on the semiconductor substrate after exposure.
Furthermore, at present, circuit patterns written on semiconductor substrates by photolithography are significantly small compared with wavelengths of exposure light. Thus, if reduction projection exposure is performed using a photomask that has a photomask pattern formed from a circuit pattern on a semiconductor substrate through 4-fold magnification is used to perform, the photomask pattern cannot be faithfully transferred onto the resist film due to interference of the exposure light or the like.
Thus, as a super-resolution mask, there are commonly used an OPC mask that corrects the optical proximity effect, which degrades the transfer characteristics, by the so-called optical proximity effect correction (OPC) and a phase-shift mask that makes the phases of adjacent aperture patterns different by 180° to make the optical amplitude at the middle of the adjacent aperture patterns zero. For example, the OPC mask has to have an OPC pattern (a hammer head, an assist bar or the like) that has a size one-half or less the size of the circuit pattern has to be formed. Besides, a half-tone phase-shift mask, which has a region transparent to the exposure light and a translucent region that has controlled transmittance and phase-shift capability, is known as a technique that provides a greatly improved resolution and widely used because it does not require significant modification of the mask design.
Typically, when forming a photomask pattern, a photoresist film is formed on a photomask blank having a light-shielding film on a transparent substrate, the photoresist film is irradiated with an electron beam to write a pattern thereon, and the photoresist film is developed to provide a resist pattern. Then, using the resist pattern as an etching mask for the light-shielding film, the light -shielding film is patterned to form a photomask pattern. In order to form a fine photomask pattern, it is important to make the photoresist film thin for the reason described below.
If the resist pattern is miniaturized without reducing the thickness of the resist film, the aspect ratio (that is, the ratio between the resist film thickness and the pattern width) of the part of the resist serving as the etching mask for the light-shielding film increases. In general, as the aspect ratio of the resist pattern increases, the pattern becomes more susceptible to degradation, and if the resist pattern is used as the etching mask, the precision of the transfer thereof onto the light-shielding film decreases. In an extreme case, a part of the resist pattern may fall or peel off, resulting in a defective pattern. Thus, as the photomask pattern becomes finer, the thickness of the resist film used as the etching mask for the light-shielding film has to be reduced to avoid an undesirably great aspect ratio. The aspect ratio is desirably equal to or less than 3. For example, when forming a resist pattern having a pattern width of 70 nm, the thickness of the resist is desirably equal to or less than 210 nm.
As for the material of the light-shielding film that is patterned using the photoresist as an etching mask, many materials have already been proposed. Among others, chrome-metal films and chromium-compound films can contain a large amount of information about etching, so that, in practical, such chromium compounds have been always used as the material of the light-shielding film, and forming the light-shielding film from the chromium compound has been substantially established as a standard process step. For example, in Japanese Patent Laid-Open Nos. 2003-195479 and 2003-195483 and Registered Japanese Utility Model No. 3093632, there have been disclosed exemplary structures of photomask blanks, which have a light-shielding film made of a chromium compound that has a light-shielding characteristic required for a photomask blank designed for ArF exposure.
In general, the light-shielding film made of a chromium compound is patterned by oxygen-and-chlorine-based dry etching. However, such etching often has a significant effect on an organic film, such as the photoresist. Thus, if the light-shielding film made of a chromium compound is etched using a relatively thin resist film as a mask, the resist is damaged during the etching, the configuration of the resist pattern changes, and it is difficult to accurately transfer the original resist pattern onto the light-shielding film.
However, it is technically difficult to make the photoresist, which is an organic film, have both high resolution and patterning precision and high etching resistance. As far as a conventional patterning process is used, there exists a tradeoff between the resolution and the etching resistance. Specifically, the photoresist film has to be made thinner to achieve higher resolution, while thinning of the photoresist film has to be limited to assure the etching resistance during the patterning step.
Thus, in order to form a highly precise photomask pattern while reducing the burden on a photoresist to reduce the thickness thereof, it is necessary to optimize the structure (including thickness and composition) of the light-shielding film to be patterned.
As for the material of the light-shielding film, many investigations have already been made. For example, in Japanese Patent Laid-Open No. 2001-312043, there is reported an example in which a tantalum metal film is used as a light-shielding film for ArF exposure. In this example, a tantalum metal film is used as a light-shielding film, a tantalum oxide film is used as an antireflection layer, and the two layers are etched using a fluorine-based gas plasma, which is relatively unlikely to damage a photoresist, in order to reduce the burden on the photoresist during etching.
However, even if such an etching condition is selected, as far as the two layers, the light-shielding film and the antireflection layer, are etched using only the photoresist as a mask, reduction of the burden on the photoresist is limited, and it is difficult to sufficiently satisfy the demand for highly precise formation of a fine photomask pattern.
Alternatively, there has been known a technique of reducing the burden on the photoresist during dry etching by using a hard mask. For example, in Japanese Patent Laid-Open No. 63-85553, there is disclosed a technique of dry-etching a metal silicide film using a SiO2 film formed on the metal silicide film as an etching mask.
However, the SiO2 film has a poor conductivity, so that a problem of charge-up tends to occur during exposure to an electron beam. Furthermore, defect inspection of the photomask blank is typically based on the reflectance thereof, and light having a wavelength of 257 nm is used for the defect inspection of a mask for ArF-exposure. In order to accurately achieve the defect inspection, the reflectance for the light of that wavelength has to fall within a range of about 10 to 20%. However, if the SiO2 film is used as an etching mask, there occurs a problem that the SiO2 film has an excessively high reflectance, which interferes with the defect inspection.
As described above, conventional photomask blank structures cannot satisfactorily meet the demand for highly precise formation of a fine photomask pattern on the light-shielding film. This problem is particularly serious in the case of a photomask for photolithography using exposure light having a short wavelength of 250 nm or less for which high resolution is required (KrF: 248 nm, ArF: 193 nm, F2: 157 nm). Thus, as the wavelength of the exposure light becomes shorter, the design of the light-shielding film for forming a highly precise photomask pattern that can reduce the burden on the photoresist becomes more important.